To study the control of a differentiation specific gene, we have isolated the chick alpha 2 collagen gene. The exons of the chick alpha 2 collagen gene are small and of similar size. This suggests that this gene arose by amplification of a single coding unit of unique size (54 b.p.) Removal of intervening sequences occurs often in more than one step by splicing at sites within introns. These internal splice sites exhibit the same sequence complementarities with a small nuclear RNA, known as U1 RNA, as the end of introns. The levels of alpha 2 collagen mRNA in the cytoplasm and those of collagen RNA precursors in the nucleus are severely reduced in Rous sarcoma virus transformed cells, strongly suggesting that control of this gene occurs at the level of stranscription. A model has been developed for the interaction of the E. coli cyclic AMP receptor subunits with their recognition sites in the gal, lac and ara promoters.